Disclosed herein is a process for the formation of polyimide resins.
A standard method to make polyimides is to employ a highly polar solvent that dissolves both the monomers and resultant polymers. Solvents such as dimethyl formamide (DMF), dimethyl acetamide (DMAC), N-methyl pyrrolidinone (NMP), hexamethyl phosphoramide (HMPA) and dimethyl sulfoxide (DMSO) are widely used in this regard. The resultant polymer is totally dissolved and can be isolated from solution by removal of solvent as part of a film casting or other evaporative process or by precipitation using an anti-solvent such as methanol.
A less widely known technique to make polyimides is described in U.S. Pat. No. 4,835,249 in which the reactants are dissolved in a solvent and then polymerized to an extent where the polymer precipitates from solution and can eventually be isolated by filtration or other related separation technique.
The aforementioned methods, while useful in some circumstances, are not universally applicable to all monomer combinations. Accordingly, there remains a need for methods of forming polyimide resins which are useful with a wider range of monomer combinations.